She is being retried for the March 2010 death of Patrick Thomas on Alma Street.
Prosecuter Jill Knee told the jury they will hear testimony that on the night of Thomas's death the couple were partying and getting along before fighting began.
Knee says the case will tell how a neighbour separated the two and MacKenzie was in a neighbour's apartment before grabbing a butcher knife and saying "I'm going to kill him" before Thomas was found bleeding from a stab wound on Alma street.
Defence lawyer David Kelly told the jury Thomas was violent and abusive when drinking and since her last trial, MacKenzie has remarried, had two more children and moved on with her life.
The trial continues at the Sydney Street courthouse